1. Technical Field
The present invention generally relates to trunk piston engine lubricating oil compositions.
2. Description of the Related Art
Trunk piston engines operate using various types and qualities of diesel fuels and heavy fuel oils. These fuels typically contain high concentrations of asphaltenes, generally the heaviest and most polar fraction of petroleum distillate. Asphaltenes are highly complex compounds believed to be composed of polyaromatic sheets containing alkyl side chains, and are generally insoluble in lubricating oils. When heavy fuel oils and conventional lubricant oil compositions mix in different temperature regions of a trunk piston engine, black sludge (such as asphaltene deposits or other deposits) and other asphaltene derived deposits (such as undercrown deposits) tend to form. The formation of black sludge or deposit can adversely affect the service interval and maintenance cost of the trunk piston engine. Therefore, an important performance aspect of these lubricants is residual fuel compatibility.
Presently, there is a move in the industry in different regions of the world to replace Group I basestocks with Group II basestocks in trunk piston engine oils. Group II basestocks generally have a lower aromatic content than Group I basestocks, thereby resulting in a loss of heavy fuel oil (also known as residual fuel oil) compatibility when Group II or higher basestocks are used in trunk piston engine lubricating oils rather than Group I basestocks. It is believed that this loss of heavy fuel oil compatibility is due to the much lower solubility of asphaltenes in the Group II or higher basestocks compared to Group I basestocks. Generally, the problem of the loss of heavy fuel oil compatibility has been typically addressed by increasing the amount of detergent-containing trunk piston engine lubricating oil additive packages.
Several attempts have been made to develop a lubricating oil composition having improved performance within trunk piston engines operating on heavy fuel oils. For example, EP 1154012 discloses a dispersant-free lubricating oil composition comprising an oil of lubricating viscosity, an overbased metal detergent, and an antiwear additive, wherein the composition can contain small amounts of a dispersant provided that the composition does not substantially demonstrate the dispersancy effect of the component.
Similarly, EP 1209218 discloses a dispersant-free lubricating oil composition comprising an oil of lubricating viscosity, an overbased metal detergent, and an antiwear additive, where the composition can contain less than or equal to 1 mass % of a dispersant.
U.S. Patent Application Publication No. 20090093387 (“the '387 application”) discloses a lubricating oil composition containing (a) a Group II basestock, and (b) a neutral or overbased metal hydrocarbyl-substituted hydroxybenzoate detergent having a basicity index of less than 2. The '387 application further discloses that the neutral or overbased metal salicylate detergent having a basicity index of less than 2 improves asphaltene dispersancy in Group II basestocks.
Accordingly, it would be desirable to provide an improved trunk piston engine lubricating oil composition which exhibits improved heavy fuel oil compatibility.